Drag to orbit the rotor · particles sediment outward from the spin axis
A centrifuge spins a sample at high speed, replacing the normal gravitational field with a much stronger centrifugal acceleration — the relative centrifugal force (RCF), measured in multiples of g — which separates particles in a liquid by size, density, and shape. The sedimentation velocity of a spherical particle under centrifugal force follows Stokes' law: v = (2r²(ρ_p − ρ_f) × ω²R) / (9η), where r is particle radius, ρ_p and ρ_f are particle and fluid densities, ω is angular velocity, R is radial distance, and η is fluid viscosity. Centrifugation underpins blood component separation in transfusion medicine, protein purification in biochemistry, uranium enrichment in nuclear technology, and cell organelle isolation in cell biology.
Adjust rotor speed (RPM), particle size, and density to watch bands of different materials form at distinct radial positions in the tube. Switch between modes — differential centrifugation, density-gradient, and isopycnic — to see how each exploits a different physical property to achieve separation.
What is RCF and how is it calculated from RPM?
Relative centrifugal force (RCF), expressed as multiples of gravity (× g), is the effective centrifugal acceleration experienced by the sample. It is calculated as RCF = 1.118 × 10⁻⁵ × r × N², where r is the rotor radius in centimetres and N is the rotor speed in revolutions per minute (RPM). A typical laboratory microcentrifuge at 12,000 RPM and a rotor radius of 8 cm generates about 10,700 × g, causing a 1-micron protein aggregate to pellet in under a minute.
What is Stokes' law and what does it say about sedimentation?
Stokes' law describes the drag force on a small spherical particle moving through a viscous fluid: F_drag = 6πηrv, where η is dynamic viscosity, r is particle radius, and v is velocity. At terminal (sedimentation) velocity, this drag balances the net gravitational or centrifugal force, giving v ∝ r² × (ρ_p − ρ_f). This means larger and denser particles sediment much faster — doubling the radius quadruples the sedimentation speed — which is why ultracentrifuges can resolve protein subunits differing by just a few kiloDaltons.
How is a centrifuge used to separate blood components?
Whole blood spun at around 2,000–3,000 × g for 10 minutes separates into three layers: a bottom red cell pellet (density ~1.10 g/mL), a thin "buffy coat" of white cells and platelets (~1.06 g/mL), and an upper plasma layer (~1.025 g/mL). Blood banks routinely perform this separation to produce packed red cells for anaemia treatment, platelet concentrates for clotting disorders, and fresh-frozen plasma. More precise fractionation of blood components, such as isolating specific immune cell populations, requires density-gradient centrifugation.
Differential centrifugation uses sequential increases in centrifugal force to pellet progressively smaller particles: nuclei at ~600 × g, mitochondria at ~10,000 × g, microsomes at ~100,000 × g. Density-gradient centrifugation layers the sample on top of a continuous or step gradient of a dense solute (sucrose or CsCl) and centrifuges until particles migrate to the position where their density matches the surrounding fluid (isopycnic) or where size-dependent velocity stops them (rate-zonal). Density-gradient methods are used to purify viruses, DNA, and protein complexes.
Ultracentrifuges operate at 150,000–800,000 × g (up to ~100,000 RPM) and can separate macromolecules by size alone. The analytical ultracentrifuge, invented by Theodor Svedberg in the 1920s, monitors the migration of molecules in real time using optical detection, allowing calculation of molecular weight and shape. Sedimentation coefficients are reported in Svedberg units (S): the 70S bacterial ribosome comprises a 50S large subunit and a 30S small subunit (the units are not additive because shape also matters).
Natural uranium contains only 0.7% of the fissile isotope ²³⁵U; nuclear reactors require 3–5% enrichment and weapons-grade material requires above 90%. Gas centrifuges spin uranium hexafluoride (UF₆) gas at ~50,000 RPM; the heavier ²³⁸U molecules migrate outward while ²³⁵U accumulates near the axis. Individual centrifuges achieve a separation factor of only ~1.3 per stage, so hundreds of cascaded machines are needed. Gas centrifuge cascades are far more energy-efficient than older gaseous diffusion plants.
The sedimentation coefficient s is the ratio of a particle's sedimentation velocity to the centrifugal acceleration: s = v / (ω²R). It has units of seconds, but typical biological macromolecules have values of 10⁻¹³ s, so the Svedberg unit (S) is defined as 10⁻¹³ s. Common reference points: DNA nucleosomes are ~11S, the 20S proteasome is ~20S, and mitochondrial ribosomes are ~55S. Svedberg values are affected by both molecular mass and shape — elongated molecules sediment more slowly than compact spheres of the same mass.
The time t to sediment a particle from the top to the bottom of a tube is approximately t = ln(r_bottom / r_top) / (s × ω²), where r_bottom and r_top are the radial distances of the tube bottom and meniscus from the rotor axis, s is the sedimentation coefficient, and ω is angular velocity in rad/s. In practice, tables or online calculators translate this formula to "run at X × g for Y minutes to pellet particles larger than Z nm," accounting for rotor geometry. Pelleting times scale inversely with RCF — doubling the speed roughly halves the time.
Catastrophic rotor failure due to imbalance, metal fatigue, or tube rupture can release energy equivalent to an explosion; rotors are enclosed in thick armoured chambers rated to contain the debris. Strict balance requirements (typically within 0.1 g difference between opposing tubes) prevent resonance oscillations that stress bearings. Biological samples require containment inside sealed rotors or aerosol-tight tubes, and spins must be performed in biosafety-rated cabinets when processing infectious material to prevent aerosol release.
In isopycnic centrifugation, the sample is mixed into a dense solute such as caesium chloride (CsCl) and centrifuged for many hours until the solute itself forms a density gradient and each particle migrates to its exact buoyant density equilibrium point. This technique was used in the 1958 Meselson-Stahl experiment to demonstrate semi-conservative DNA replication by resolving ¹⁴N-DNA from ¹⁵N-DNA (difference of only ~0.014 g/mL) in CsCl gradients at 140,000 × g.